The antibacterial effect of cationic surfactants against the pure culture of phosphate (P)-accumulating bacterium Acinetobacter junii was investigated. The estimated EC50 values of the N-dodecylpyridinium chloride (DPC) for growth inhibition was 1.4&plusmn ; 0.5 x 10-6 mol L-1 and for the inhibition of the P-uptake rates 7.3&plusmn ; 2.6 x 10-5 mol L-1. The estimated EC50 values of the N-cetylpyridinium chloride (CPC) for growth inhibition was 4.9&plusmn ; 1.3 x 10-7 mol L-1 and for the inhibition of the P-uptake rates 7.7&plusmn ; 2.9 x 10-6 mol L-1. This suggests the importance of controlling the amounts of cationic surfactants in influent of the wastewater treatment systems in order to avoid the possible failure of the biological P removal from wastewaters.